Measurements of riverine dissolved inorganic carbon (DIC), total alkalinity (AT), pH, and the partial pressure of carbon dioxide (pCO2) can provide insights into the biogeochemical function of rivers including the processes that control biological production, chemical speciation, and air-water CO2 fluxes. The complexity created by these combined processes dictates that studies of inorganic carbon be made over broad spatial and temporal scales. Time-series data like these are relatively rare, however, because sampling and measurements are labor intensive and, for some variables, good measurement quality is difficult to achieve (e.g., pH). In this study, spectrophotometric pH and total alkalinity (AT) were quantified with high precision and accuracy at biweekly to monthly intervals over a four year period (2018-2021) along 216 km of the Upper Clark Fork River (UCFR) in the northern Rocky Mountains, USA. We use these and other time-series data to provide insights into the processes that control river inorganic carbon, with a focus on pCO2 and air-water CO2 fluxes. We found that seasonal snowmelt runoff increased pCO2 and that expected increase and decrease of pCO2due to seasonal heating and cooling were likely offset by an increase and loss of algal biomass, respectively. Overall, the UCFR was a small net source (0.08 ± 0.14 mol m-2 d-1) of CO2 to the atmosphere for all four years of our study with highly variable annual averages. The highly dynamic, seasonally correlated, offsetting mechanisms highlight the challenges in predicting pCO2 and air-water CO2 fluxes in rivers.

Less than three decades ago only a small fraction of the Arctic Ocean (AO) was ice free and then only for short periods. The ice cover kept sea surface COat levels lower relative to other ocean basins that have been exposed year round to ever increasing atmospheric levels. In this study, we evaluate sea surface COmeasurements collected over a 6-year period along a fixed cruise track in the Canada Basin. The measurements show that mean COlevels are significantly higher during low ice years. The COincrease is likely driven by ocean surface heating and uptake of atmospheric COwith large interannual variability in the contributions of these processes. These findings suggest that increased ice-free periods will further increase sea surface CO, reducing the Canada Basin’s current role as a net sink of atmospheric CO.